The addictions are highly heritable neuropsychiatric diseases; however, the responsible genetic factors remain largely unknown. Both an individual's genotype and the particular environment can powerfully influence the subjective and physiological response to drugs of abuse. Accordingly, gene x environment (G x E) interactions are hypothesized to contribute substantially to the heritability of the addictions. Mice serve as a important model organism for understanding the genetic basis of G x E interactions, permitting experimental control over genetic and environmental factors. The primary objective of this proposal is to identify the genetic basis of G x E interactions in behavioral traits that accompany the progressive stages of addiction. We observed a potent G x E interaction in the rewarding effect of the widely abused opioid oxycodone (OXY) in C57BL/6J (B6J) and C57BL/6NJ (B6NJ) substrains in the conditioned place preference (CPP) test. B6NJ showed up to a five-fold increase in OXY-CPP relative to B6J, but only when two out of four cage mates received OXY during training. In contrast, when all four mice received OXY, the effect of genotype was eliminated. Thus, the post-training social drug environment of the home cage can exert divergent effects on drug seeking behavior, depending on the genotype of the mice. We developed a multi-stage addiction assessment protocol (MSAAP) to measure a panel of addiction traits for G x E interactions in B6 substrains, including reward, analgesic tolerance, and the emotional-affective component of opioid withdrawal. In addition to substrain differences in opioid reward, preliminary studies indicate a three-fold increase in the emotional-affective component of OXY withdrawal in B6NJ versus B6J in the elevated plus maze. Because B6 substrains are nearly genetically identical, mapping the genetic basis of G x E interactions for MSAAP traits is expected to yield quantitative trait loci (QTLs) that possess very few predicted functional variants. In support, preliminary studies demonstrate that a B6J x B6NJ- F2 Reduced Complexity Cross (RCC) can be used to identify QTLs for diverse complex traits that contain fewer than 10 predicted functional variants per locus. In Aim 1, the RCC will be used to map the genetic basis of G x E interactions in MSAAP traits and select the most likely candidate genes based on the limited number of predicted functional variants that are expected to underlie the QTLs. In Aim 2, candidate genes will be validated using transcription activator-like endonucleases (TALENs) to introduce targeted null mutations and assess their role in G x E interactions in addiction traits. The identification of novel genetic factors mediating G x E interactions in multiple facets of addiction will enhance our understanding of the dynamic neurobiological mechanisms that support its progression. The results of these studies support the long-term goal of developing novel preventative and treatment strategies for addiction. An attractive notion is that clinicians will one day determine the most appropriate treatment based on both the patient's genotype and historical environment of drug abuse.